1. Field of the Invention
This invention relates to improved crosslinked polymeric compositions used as insulation for wire and cable products. More particularly, the invention relates to flame-retardant ethylene-alkyl acrylate copolymer formulations used as wire and cable insulation for applications where high service temperatures are required.
2. Description of the Prior Art
Fire resistant polyolefin compositions are widely used for wire and cable insulation. In electrical environments both insulating and fire resistant properties are considered to be essential. Additionally, the compositions must be readily processable and should not deteriorate under the service conditions.
A widely used fire retarding insulation for wire and cable is comprised of a crosslinkable polymer, such as polyethylene or ethylene-vinyl acetate copolymer, one or more stabilizers or antioxidants, one or more hydrated inorganic fillers, and a crosslinking agent. Other additives such as pigments, processing oils, lubricants and coupling agents can also be included in these formulations. Compositions of this type which find use as single layer insulation and jacketing for copper wire are disclosed in U.S. Pat. Nos. 3,832,326 and 3,922,442 to North et al., and U.S. Pat. Nos. 4,349,605 and 4,381,362 to Biggs, et al.
For applications involving high service temperatures, such as 150xc2x0 C. rated wires for use in ovens, water heaters, dryers, toasters, cookers and xe2x80x9cunder-the-hoodxe2x80x9d automobile uses, numerous compositions based on crosslinked polyethylene were developed capable of meeting the tensile strength and percent elongation retention requirements under the initial test criteria which specified aging at 158xc2x0 C. for 90 days. However, as test conditions became more stringent in recent years, extending the aging period to 150 days in UL Standard 1581 (Style 3321), few of these original formulations were capable of meeting the more rigorous test conditions particularly when copper wire was being insulated. Copper has been reported to catalyze the auto-oxidation of polymers (see Z. Osawa, Polym. Deg. And Stab., 20, 203-236 (1988)) and, at the elevated temperatures encountered in severe service applications, it is believed copper I and II ions further accelerate the decomposition of polymer hydroperoxides to chain propagating radical species.
Accordingly, there is a need for effectively stabilized flame retardant insulation compositions which are readily processable yet capable of retaining tensile and elongation properties upon long term aging at elevated temperatures, particularly in the presence of copper.
The use of benzimidazoles to stabilize polyolefin compositions, particularly polyethylene and polypropylene, is known. U.S. Pat. No. 3,218,276 discloses the use of alkyl benzimidazole to stabilize fiber-forming polyolefins. Polypropylene fiber-forming compositions containing 0.2 to 2.0 percent benzimidazole with other conventional additives are disclosed. U.S. Pat. No. 2,997,456 teaches the use of metallic mercaptobenzimidazole compounds as stabilizers for polymers of 1-olefins, primarily polypropylene, to protect against molecular degradation under conditions of elevated temperature and/or mechanical working and zinc mercaptobenzimidazole is specifically mentioned.
The use of combinations of hindered phenols with various zinc salts of mercapto compounds to provide stabilization of cured and crosslinked polyolefins utilized as insulation for electrical conductors is disclosed in U.S. Pat. Nos. 4,260,661, 4,693,937, 4,797,323 and 4,824,883. For example, combinations of IRGANOX 1010 with the zinc salt of 2-mercaptobenzimidazole (ZMB), the zinc salt of 2-mercaptotolylimidazole (ZMTI) and the zinc salt of 2-mercaptobenzothiazole (ZMBT) are all illustrated. U.S. Pat. No. 4,459,380 discloses combining a sterically hindered phenol with a zinc salt of a mercaptoimidazole to stabilize crosslinkable curable ethylene-propylene rubber compositions. All of the references provide for the inclusion of other conventional additives, such as Sb2O3, halogenated compounds, fillers, silanes and crosslinking agents in the formulations. It is mentioned that ethylene copolymers, including ethylene-acrylate copolymers, can be stabilized using these zinc salt/hindered phenol combinations. U.S. Pat. No. 5,196,462 also shows the use of these combinations to stabilize thermoplastic elastomers and indicates that other antioxidants, such as phenols, thiodipropionates and quinolines may also be present.
Rubber/silicone compositions containing a metal benzimidazole, an aromatic secondary amine, an organopolysiloxane oil and organic peroxide are disclosed in U.S. Pat. No. 4,808,643.
It is an object of the present invention to provide improved crosslinkable, flame retardant polymeric compositions based on ethylene-alkyl acrylate copolymers which have good processing characteristics and are resistant to oxidative degradation. It is a further objective to provide compositions useful as insulation for wire and cable, particularly applications involving exposure to high service temperatures. The compositions of the invention meet UL Standard 1581 and, more specifically, UL Subject 758, Style 3321.
In accordance with this invention, the above objectives are realized utilizing an ethylene-alkyl acrylate base resin with a stabilizer consisting of a mixture of a zinc salt of a mercaptobenzimidazole and secondary aromatic amine compound at prescribed ratios, a brominated flame retardant, antimony trioxide, a hydrated inorganic filler and a chemical crosslinking agent. More specifically, the formulations comprise (1) 30 to 65 weight percent of a copolymer of ethylene and 3 to 40 weight percent alkyl acrylate having the formula 
wherein Rxe2x80x2 is C1-4 alkyl and Rxe2x80x3 is hydrogen or methyl having a melt index of 0.1 to 15 g/10 mins; (2) 1 to 10 weight percent of a stabilizer consisting of a mixture of a zinc salt of a mercaptobenzimidazole of the formula 
where R is a C1-4 alkyl group and n is 0 to 4 with a secondary aromatic amine, the ratio of the zinc mercaptobenzimidazole compound to the secondary aromatic amine compound ranging from 0.2:1 to 20:1; (3) 5 to 40 weight percent brominated aromatic flame retardant compound; (4) 1.5 to 20 weight percent antimony trioxide; (5) 5 to 50 weight percent hydrated inorganic filler; and (6) 0.1 to 4 weight percent chemical crosslinking agent.
Ethylene-n-butyl acrylate copolymer formulations stabilized using a combination of ZMTI or ZMB with 4,4xe2x80x2-bis(xcex1,xcex1-dimethylbenzyl)diphenyl amine are highly useful for the invention. Compositions of the above type containing 42 to 52 weight percent base resin, 2.5 to 9 weight percent of the stabilizer combination, 10 to 15 weight percent brominated aromatic flame retardant, 3 to 6 weight percent antimony trioxide, 20 to 35 weight percent magnesium hydroxide and 1 to 2 weight percent tertiary organic peroxide are an even more preferred embodiment. Most preferred and highly useful for 150xc2x0 C.-rated appliance wire applications are compositions wherein the copolymer base resin has a melt index of 0.3 to 10 g/10 min and contains 10 to 30 weight percent n-butyl acrylate; the brominated aromatic flame retardant is ethylene bistetrabromophthalimide; the hydrated inorganic filler is magnesium hydroxide; and the tertiary organic peroxide is selected from the group consisting of dicumyl peroxide and xcex1,xcex1xe2x80x2-bis(t-butylperoxide)diisopropylbenzene.